The invention relates to a color display device comprising a color cathode ray tube including an in-line electron gun for generating three electron beams, a color selection electrode and a phosphor screen on an inner surface of a display window and a means for deflecting the electron beams across the color selection electrode.
Such display devices are known.
The aim is to make the outer surface of the display window flatter, so that the image represented by the color display device is perceived by the viewer as being flat. However, an increase of the radius of curvature of the outer surface will lead to an increase of a number of problems. The radius of curvature of the inner surface of the display window and of the color selection electrode should also increase, and, as the color selection electrode becomes flatter, the strength of the color selection electrode decreases and hence the sensitivity to doming and vibrations increases. An alternative solution to this problem would be to curve the inner surface of the display window more strongly than the outer surface. By virtue thereof, a shadow mask having a relatively small radius of curvature can be used. As a result, doming and vibration problems are reduced, however, other problems occur instead. The thickness of the display window is much smaller in the center than at the edges. As a result, the weight of the display window increases and the intensity of the image decreases substantially towards the edges.
It is an object of the invention to provide a color cathode ray tube of the type mentioned in the opening paragraph, in which the outer surface may be flat or almost flat, while, at the same time, the above problems are overcome or reduced.
To achieve this, a color display device in accordance with the invention is characterized in that the color display device comprises a first and a second means, which are arranged at some distance from each other to dynamically influence the trajectories of the electron beams, to decrease the distance between the electron beams at the location of the deflection plane as a function of the deflection in at least one deflection direction.
The color display device in accordance with the invention has a first and a second means, arranged at some distance from each other, for dynamically influencing the trajectories of the electron beams. By virtue thereof, the distance between the electron beams (also referred to as xe2x80x9cpitchxe2x80x9d) in the plane of deflection can be changed dynamically in such a manner that this distance decreases as the deflection increases. By dynamically changing this distance, as a function of the deflection, and hence as a function of the x and/or y-coordinate(s), the distance between the display window and the color selection electrode can increase accordingly in the relevant deflection direction. The shape of the inner surface of the display window and the distance between the display window and the color selection electrode determine the shape, in particular the curvature, of the color selection electrode. Since the distance between the electron beams decreases as a function of the deflection, the distance between the display window and the color selection electrode increases and the shape of the color selection electrode can deviate more from the shape of the inner surface of the display window than in known cathode ray tubes, and, in particular, its curvature can be greater. Said greater curvature (in other words, a smaller radius of curvature) increases the strength of the color selection electrode and reduces doming and microphonics.
Preferably, the first means is integrated in the electron gun, that is, the first means comprises one or more components of the electron gun.
In comparison with a separate first means, this has the advantage that fewer components are necessary and that the distance between the first and the second means is increased, thus enabling an increase of the possible variation in distance between the electron beams and hence of the variation in distance between the color selection electrode and the display screen and, consequently, a greater change in curvature of the color selection electrode.
Preferably, the first means comprises one or more components of the prefocusing portion of the electron gun. As a result, the distance between the first and the second means is increased, compared to embodiments in which the first means is situated at the location of, for example, the main lens portion, thus enabling an increase of the possible variation in distance between the electron beams and hence of the variation in distance between the color selection electrode and the display screen.
Alternatively, in embodiments a separate first means is used. The advantage of using a separate first means is that the electron gun design need not be changed. Since the electron gun design need not be changed, the electron-optical functions of the electron gun such as the generation, beam forming and focusing of the electron beams are not or hardly affected by the introduction of the first means, and application of a separate first means is much easier. Preferably the separate first means are situated on the outside of the envelope. The means are then easily accessible, and current can easily be supplied.
Preferably, the second means is integrated in the deflection means, that is, the second means comprises one or more components of the deflection means.
This has the advantage, compared to a separate second means, that fewer components are necessary and that the distance between the first and the second means is increased, thus enabling an increase of the possible variation in distance between the electron beams and hence of the variation in distance between the color selection electrode and the display screen.
Preferably, the distance between the electron beams as a function of the deflection varies at least 2%. As a result, the radius of curvature of the color selection electrode can change so much that a noticeable change in doming and microphonic properties is achieved. In a further preferred embodiment, the distance between the outer beams varies more than 5%. This enables a greater change in radius of curvature to be achieved, which has a strong influence on doming and microphonic properties.
These and other objects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.